Vitamin D is also known as the ‘Sunshine’ vitamin or ‘Sunny D’ although classified as a fat soluble vitamin this nutrient is actually a hormone produced naturally when the skin is exposed to sunlight. The hormone is produced in the kidneys and can be found in some food sources. It is needed for calcium absorption and regulate blood calcium concentration. Vitamin D also has an important role in the immune system. The two forms or compounds of vitamin D include Vitamin D2 (Ergocalciferol / calciferol) and Vitamin D3 (Cholecalciferol). Vitamin D2 is the form found in vegetable sources of food such as mushrooms.
Vitamin D3 is the natural form of Vitamin D found in fish, fish oils, egg yolks and produced through the skin when exposed to sunlight or ultraviolet rays. Vitamin D3 (cholecalciferol) is made from 7-dehydrocholesterol in the skin when exposed to sunlight and was first discovered in 1935. The word cholecalciferol comes from the Greek word ‘kholḗ’ which refers to the bile. Chole is also found in the word cholesterol with sterol from the Greek word ‘stereos’ meaning stiff. Cholesterol itself is a solid substance found in the body and needed to make vitamin D from the skin. Calci comes from the word calciferous and refers to calcium salts such as calcium carbonate.
The synthesis of vitamin D is controlled by calcium, phosphate and calcitriol. Parathyroid hormone is involved in the production of active vitamin D by promoting the activity of the necessary enzyme (catalyst) that helps with catabolize inactive D to active D. 
Calcifediol (25-hydroxyvitamin D) is a hormone precursor that is produced in the liver by hydroxylation of vitamin D3. Calcitriol (1,25 dihydroxyvitamin D) is the active form of vitamin D and it is converted to this form by the liver and kidney. Both vitamin D2 and vitamin D3 are converted to calcifediol and then to calcitriol. Calcifediol and calcitriol levels can both be measured to determine vitamin D levels in a blood test and that the conversion to the active form is adequate.
The main functions of vitamin D include maintaining bone health working synergistically with other bone promoting nutrients which includes calcium, magnesium and phosphorus.
Most people can synthesise vitamin D from sunlight and it can be obtained through dietary intake or supplements.
Maintain bone health and teeth
Vitamin D is one of the most important nutrients for bone health as its main role is to regulate the absorption of other nutrients needed for the health of the bones and teeth which includes calcium, magnesium and phosphorus.
A deficiency can cause many bone disorders such as osteoporosis, rickets and osteomalacia. Having sufficient vitamin D levels helps maintain strong bones and prevent bone related disorders such as osteoporosis most common in the elderly. It can also help post-menopausal women to maintain bone health.
Research is showing that vitamin D intake from diet and supplements may help reduce bone loss in those who are susceptible and at high risk to bone disorders such as the elderly and post-menopausal women.
In addition to bone health vitamin D also helps maintain joint health. A deficiency of vitamin D is common in those who are afflicted with rheumatoid arthritis. 
In addition to vitamin D, vitamin K also acts as a co factor for maintaining bone health and a combination can help reduce the risk of fractures and other bone related disorders.
Vitamin D works synergistically with calcium to help strengthen bone mineral health.
Supports brain health and prevents depression
Vitamin D has role in regulating various hormones and neurotransmitters essential for brain health and preventing depression. These include adrenaline, epinephrine, norepinepherine (noradrenaline), dopamine and serotonin. The adrenal gland regulates tyrosine hydroxylase which is the rate limiting enzyme involved in the production of neurotransmitters which is achieved by the gland with activated vitamin D.
A deficiency of this vitamin can lead to depression and seasonal affective disorder (SAD).
May help prevent cancer
One study has shown estrogen levels reduced by 3% and progesterone by 10% after administering 24,000 IU per week for a four week duration sampled from the luteal phase suggesting that high doses may reduce the risk of breast cancer risk. 
Hyperparathyroidism is also associated with the risk of breast cancer. Hyperparathyroidism, when the parathyroid glands produce too much parathyroid hormone due to benign tumor growth near the parathyroid glands (parathyroid adenoma), causes calcium levels in the blood to increase possibly by taking the calcium from the bones. High calcium levels and parathyroid hormone levels can be shown in a blood test. Hyperparathyroidism has been associated with low levels of vitamin D. 
Cancer is one of the most common cause for elevated levels of calcium. Cancers that may cause hypercalcemia include cancer of the lungs, head and neck, kidney, breast, gastrointestinal cancers and multiple myeloma. Metastasis which is when cancer spreads to the bones and also increases the risk of hypercalcemia.
Vitamin D deficiency has been linked to various types of cancer and studies are showing the Sunshine vitamin may protect against cancers.
Support the immune system
Vitamin D helps build the immune function and improves immune cell production. Many autoimmune disorders and infectious diseases are usually due to low levels of vitamin D. A sufficient supply of vitamin D along with other immune system promoting nutrients such as vitamin C and zinc can reduce symptoms of the common cold and other infections.  
Preventative for autoimmune disorders
Many of the autoimmune disorders have been linked to a deficiency of vitamin D. These include rheumatoid arthritis, thyroiditis multiple sclerosis (MS), sjogren’s syndrome, and crohn's disease.
Help manage diabetes and support cardiovascular health
Vitamin D deficiency has been linked to low levels of insulin and insulin resistance.
Having a sufficient supply of vitamin K and D can improve insulin levels and blood pressure. Vitamin K is a co factor for vitamin D and helps by preventing build-up of calcium in the blood and reducing calcification of soft tissues. This in addition reduces the risk of arthrosclerosis by helping calcium being deposited in the bones. Improved insulin levels can help manage diabetes and reduce the risk of cardiovascular disease. Insulin resistance is associated with heart disease which is more common in northern climates.
Support fertility and reduce PMS
Low levels of vitamin D has been linked to infertility. Vitamin helps support calcium and magnesium supplies in the body. When sufficient and appropriately utilized these minerals help relieve PMS symptoms.
Vitamins: A, B2, B3, B12, C, E, K
Minerals: B, Ca, Cu, Mg, Mn, Na, P, Se
Other: Sunlight (UV rays), Cholesterol, Lycopene, Parathyroid hormone (PTH), largest meal of the day
Taking vitamin D with the largest meal of the day improves absorption with a 50% increase in serum levels. 
Vitamin K works synergistically with vitamin D by promoting calcium deposits in the bones and effectively reducing calcium deposits in blood vessels reducing the risk of arthrosclerosis.
Sufficient vitamin K can help problems associated with high levels of vitamin D.
Vitamin D works in the body by maintaining normal levels of calcium and phosphorus in the blood.
Vitamin D and A work synergistically when levels are normal, at higher levels they are antagonistic
Magnesium is a cofactor for vitamin D biosynthesis, transport, and activation 
Magnesium intake can reduce risks of vitamin D deficiency 
Vitamin D can increase serum concentrations of magnesium. 
Vitamin D can increase serum level of selenium. 
Vitamins: A, K, E
Minerals: Ca, Cu, K, Mg, P, Zn
Other: Sunscreens, low sun exposure, certain medications
High amounts of phosphorus can reduce vitamin D levels
High levels of vitamin A can reduce vitamin D uptake 
Vitamin E and K follow the same pathway needed for vitamin D uptake and may compete for absorption.
Wilson’s disease from excess copper may alter vitamin d metabolism 
Vitamin D3 can be produced naturally when the skin is exposed to sunlight and can also be found in animal based food sources which includes fish oils, canned sardines, salmon, tuna, liver, cheese and egg yolks.
Vitamin D2 is found naturally in some mushrooms. Some varieties of mushrooms synthesise large amounts of vitamin D when exposed to UV rays after harvest. 
Some foods and drinks may be fortified with vitamin D2 such as milk, orange juice, breakfast cereals, yogurt, soy milk and rice milk. This will usually be indicated on the label.
The amounts recommended for vitamins and minerals are different for each country which can be found on the government website of host country. The nutritional data for each country are based on scientific research which are presented by the scientific academies in each country to help advise governmental departments for food and human nutrition.
Below is a list of some of the countries and the EU for which nutritional guidelines are available found on corresponding official government website (including the national academy of science website for the USA).
Australia and New Zealand
Vitamin D can be found in a variety of supplemental forms including capsules, gelatine capsules, tablets, liquids and can also be found in multivitamin formulas.
Vitamin D is available as vitamin D2 and vitamin D3. Studies have shown that vitamin D3 is easier to absorb as it is closest to the natural form produced in the body and also found in animal based foods.
Vitamin D in sunflower or vegetable oil base are not usually the best types as the oils are prone to going rancid which when consumed can increase free radicals in the body. Olive oil or coconut oil based vitamin D3 are less likely to go rancid although the olive oil containing vitamin D should be refrigerated to increase its shelf life.
Most supplements come with the International Unit measurement on label although it should also contain the mcg equivalent usually followed by a mcg symbol ‘μg’.
1 IU equals 0.025 mcg, to convert from IU to mcg of vitamin D, simply multiply 0.025 by the IU. As an example to find out the micrograms of 1000 IU we can multiply this by 0.025 (1000 X 0.025) which will give 25 mcg. Another method is to divide the IU amount by 40.
Vitamin D deficiency is more common in northern climates and in those who get less sunlight which usually may require supplementation. Vitamin D deficiency is becoming more prevalent and the following are symptoms of a deficiency:
- Tiredness (Fatigue)
- Poor immune system
- Impaired Wound Healing (Slow or delayed)
- Chronic infections
- Head sweating
- Cold / Flu
- Bone Pain
- Muscle Pain
- Back Pain
- Chronic pain (often in your bones)
- Bone Loss
- Hair Loss
- Gut issues
- Bones that break easily
- Weak muscles
- Muscle spasms
- Muscle cramps
- Dry eye
- Impaired tear function
- Hypocalcemia (low serum calcium)
- Cardiovascular disease
The following are some of the causes of vitamin D deficiency:
- Working indoors
- Avoiding sun exposure
- Using sunscreen protection
- Lack of co factor fat soluble vitamins such as A and K
- Vegan or strict vegetarian diets
- Being older
- Having darker skin
- Being overweight
- High muscle mass and low fat
- Overly protecting yourself from the sun
- Not going outside often
- Impaired kidney function
- Digestive issues causing malabsorption
- Nothern climates (seasonal changes)
- Poor weather conditions (cloud, smog)
A vitamin D deficiency can increase the risk of infections, cancers, multiple sclerosis and increase risk of death from cardiovascular disease.
Sunlight exposure is needed for the skin to naturally synthesise vitamin D. Lack of exposure to sunlight is one of the main causes of vitamin D. A deficiency of vitamin D is treated with supplements.
A simple finger prick blood test is available for determining vitamin d levels. Vitamin D blood test usually checks for the blood serum of 25-hydroxy vitamin D (calcifediol). Healthy levels range between 20 ng/mL (nanograms/millilitre) to 50 ng/mL. Less than 12 ng/mL is a sign of vitamin D deficiency.
Calcifediol and calcitriol can both be checked on a blood test. Calcifediol is the inactive form of the vitamin. For some individuals it may be necessary to check for calcitriol (1,25 dihydroxyvitamin D) which is the active form calcifediol is converted into to ensure that the body is using the vitamin D.
Certain health conditions such as diabetes and hyperparathyroidism have been linked to vitamin D deficiency. Vitamin D deficiency also increases the risk of death from cardiac disease.
Vitamin D toxicity is possible through excessive intake of supplements. Synthesising vitamin D naturally in the body when the skin is exposed to sunlight does not lead to toxicity as the body is able to control the amount produced.
The following are signs and symptoms of toxicity:
- Poor appetite
- Stomach pain
- Loss of appetite
- Weight loss
- Impaired kidney
- High blood levels of calcium (hypercalcemia)
- High blood pressure
- Frequent urination
- Kidney stones
- Calcification of soft tissue (arteriosclerosis)
You can read about hypercalcemia by clicking here
Taking excessively high doses of 60,000 international units (IU) a day of vitamin D for several months has shown to cause toxicity. This amount exceeds the recommended dietary allowance. Therapeutic doses of high amounts should only be considered under the guidance of your physician as it important to constantly check your levels and once sufficient to then return to a standard dose for maintenance.
It is important to discuss with your doctor or other health professional before supplementing with vitamin D as it may interact or inhibit the effects of various medications or treatments.